1. Field of the Invention
The present invention relates generally to ventricular assist devices, and particularly to a system and method for powering and controlling a pneumatic, in-series left ventricular assist device used to assist patients experiencing heart failure.
2. Description of the Related Art
Pneumatic, in-series left ventricular assist devices can be grouped into two classes according to their intended duration of use: temporary, in use for a few hours to a few weeks; and permanent, in use for the remaining life of the patient.
The temporary version is more commonly known as an intraaortic balloon pump (IABP). It has been in widespread clinical use since about 1967. The permanent version has undergone extensive experimental and limited clinical use. It is referred to alternatively as a mechanical auxiliary ventricle (MAV) or a dynamic aortic patch.
Both classes of assist device involve the placement of an avalvular pumping bladder into the descending thoracic aorta of the patient. To accomplish its pumping function, the pumping bladder is cyclically inflated synchronously with the natural heart beat. Inflation of the pumping bladder, with a shuttle gas such as helium, is timed to occur immediately after the aortic valve has closed. The volume of blood displaced as the pumping bladder is inflated causes an increase in diastolic blood pressure and, consequently, an increase in coronary artery blood flow. This increased blood flow increases the oxygen supply available to nourish the heart muscle. Later in the heart cycle, just prior to the opening of the aortic valve, the shuttle gas is exhausted from the pumping bladder causing a drop in systolic blood pressure which leads to a decrease in the oxygen requirement or demand of the heart and an increase in the amount of blood pumped. Thus, in-series left ventricular assistance increases mean blood pressure, increases blood flow, (i.e., cardiac output) and improves the oxygen supply/demand ratio in the heart.
The magnitude of these beneficial changes, and therefore the amount of hemodynamic assistance the patient receives, depends on the displacement volume of the pumping bladder and the accuracy with which the timing of the inflation/deflation cycle matches the opening and closing of the aortic valve. With regard to the first, maximum pumping bladder displacement is limited by anatomical considerations. With regard to the second, accuracy of timing in a conventional IABP system is sought in the following manner. The patient's ECG is monitored to detect occurrence of the QRS wave and it is assumed that the opening of the aortic valve follows the occurrence of the QRS wave by a fixed time interval. The closing at the aortic valve is assumed to follow by a somewhat longer, fixed time interval. These time intervals are estimated by an operator while looking at the displayed aortic pressure. The controller monitoring the electrocardiogram detects the occurrence of each QRS wave and uses the operator estimated time intervals to control the inflation and deflation of the pumping bladder.
The newest, commercially available IABP systems make one small improvement in the system described above. These systems assume that the time delays between the QRS wave and the opening and closing of the aortic valve are a function of the heart rate. A family of function curves relating heart rate to time delays from occurrence of the QRS wave to opening and closing of the aortic valve has been determined statistically from a large number of patients. However, in these newest systems the operator is still required to estimate the time intervals between the QRS wave and opening and closing of the aortic valve at the patient's present heart rate. The system then selects a specific member of the family of function curves based on the operator estimates of the noted time intervals.
Such statistically based information is, however, correct only on the average. For any specific patient the estimates are usually wrong. Furthermore, even for one specific patient at one specific heart rate the time intervals vary depending on the patient's present hemodynamic condition. Patients in whom an intraaortic balloon pump is placed are usually hemodynamically unstable. Thus, adjustment of timing of inflation and deflation of the pumping bladder may be necessary even if the patient's heart rate remains constant.
Therefore, contrary to labels otherwise, timing of inflation and deflation in prior art IABP and MAV systems is manual, and the responsibility of the operator. Arrhythmia, essentially random variation of instantaneous heart rate, makes accurate timing of inflation and deflation more difficult, even when a conscientious, well trained operator is present. The operator simply cannot react fast enough to adjust machine settings to each individual heart beat, thereby diminishing the effectiveness of the assist system.
It is therefore an object of the present invention to provide an automatic IABP system which converges on optimal timing adjustments for any heart rhythm, for any patient, without operator involvement.
It is another object of the present invention to provide an MAV system wherein a practical, long term solution for patients with heart failure is possible by enabling the system to work safely and effectively without operator supervision while the patient goes about his or her daily life at home, at work, at leisure, or in transit.
It is yet a further object of the present invention to provide an IABP system which is usable in localities where highly trained operators are unavailable.
It is yet another object of the present invention to expand the usefulness of IABP systems by maximizing their effectiveness under all possible conditions.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.